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Abstract—In this research, the degree of pozzolanic reaction
of fly ash in blended Portland cement pastes cured at different
temperatures was determined by the selective dissolution
method. The effect of curing temperature on pozzolanic
reaction was then investigated using the modified Jander’s
model. The results confirm that the pozzolanic reaction of fly
ash is strongly influenced by curing temperature and
replacement ratio of fly ash. The higher the curing temperature
and the lower the fly ash replacement ratio, the higher is the
degree of pozzolanic reaction of fly ash. The rate and
mechanism of pozzolanic reaction of fly ash vary with curing
temperature. Elevated curing temperatures lead to faster the
onset and accelerated the rate of the main reaction linearly.
Index Terms—Fly ash, pozzolanic reaction, cement paste,
curing temperature, hydration.
I. INTRODUCTION
Fly ash, a by-product from coal combustion process, is
widely used as a supplementary cementitious material (SCM)
in high performance concrete (HPC). Partially replacing the
Portland cement with fly ash has been reported to increase
long-term strength and durability of the resulting concrete [1],
[2]. This can be attributed to the pozzolanic reaction between
fly ash and calcium hydroxide (Ca(OH)2) produced by
hydration of the cement. The pozzolanic reaction produces
additional calcium silicate hydrate (C-S-H) product to fill up
capillary pores, making the fly ash concrete denser in
microstructure as compared to normal concrete [1], [3].
Therefore, the kinetics of pozzolanic reaction of fly ash is
crucial information to understand the microstructure
development and to predict the long-term performances of
the fly ash concrete.
The kinetics of pozzolanic reaction of fly ash is known to
be influenced by numerous factors, such as chemical and
physical properties of the fly ash particles, water to binder
ratio, replacement ratio of fly ash, and curing temperature.
Previously, the effect of these factors has been investigated
by researchers [3], [5]-[7]. However, the effect of curing
temperature is relatively unclear.
In real application, the hydrating fly ash concrete might be
subjected to temperature variations in various situations. For
example, the core of large fly ash concrete element is
subjected to a couple of days of temperature rise due to heat
Manuscript received September 8, 2013; revised November 10, 2013.
Mongkhon Narmluk is with the Department of Civil Technology
Education, Faculty of Industrial Education and Technology, King Mongkut’s
University of Technology Thonburi, Bangkok, Thailand (e-mail:
mongkhon.nar@kmutt.ac.th).
Toyoharu Nawa is with the Division of Sustainable Resources
Engineering, Graduate School of Engineering, Hokkaido University, Japan.
II. MATERIALS AND METHODS
A. Materials
Ordinary Portland cement (OPC) and fly ash with the
chemical and mineralogical composition in Table I and Table
II were used in this study. As can be seen from Table I, the fly
ash contains 3.36% CaO and 88.15% of (SiO2+Al2O3+Fe2O3),
classifying this fly ash as a low calcium fly ash (ASTM
C618-05, 2005). The mineralogical compositions in Table II
show that the fly ash consists of 82% of amorphous phases
and the remaining portions consist of mulite, quartz, and
hematite.
B. Sample Preparations
Two fly ash-cement pastes with different fly ash volume
fractions were produced with a constant w/b ratio of 25 % (by
weight). In the following, these two mixtures will be referred
to as FA25, and FA50, respectively. Details of mixture
proportions are shown in Table III.
To prepare test specimens, cement and fly ash powders
were mixed at room temperature until homogeneity of the
mixture was obtained. Then mixing water containing
Effect of Curing Temperature on Pozzolanic Reaction of
Fly Ash in Blended Cement Paste
Mongkhon Narmluk and Toyoharu Nawa
31
International Journal of Chemical Engineering and Applications, Vol. 5, No. 1, February 2014
DOI: 10.7763/IJCEA.2014.V5.346
of hydration. In such situations, the early age kinetics of
pozzolanic reaction of fly ash can be remarkably different
from that at room temperature. Therefore, in modeling of fly
ash concrete performances, the effect of temperature on the
pozzolanic reaction of fly ash should be taken into account.
However, at present, a clear understanding regarding the
quantitative effect of temperature on the reaction rate and
mechanism of pozzolanic reaction of fly ash has not been
well established, especially in HPC with low w/b ratio. There
is very limited research work describing the effect of curing
temperature on the kinetics of pozzolanic reaction of fly ash
in actual fly ash-cement paste [7], [8]. Hanehara et al. [7]
reported that the onset of the pozzolanic reaction of fly ash at
the curing temperature of 20 oC is at 28 days or longer, and
the pozzolanic reaction of fly ash in cement paste highly
depends upon the curing temperature. Although, this work
provided a perspective view, it focused on long-term kinetics
and no kinetics analysis has been presented.
The main objective of this paper is to provide experimental
evidences of the temperature dependency of the reaction
kinetics of fly ash in low water to binder (w/b) fly ash-cement
paste. The degree of pozzolanic reaction of fly ash was
measured as a function of curing ages by means of the
selective dissolution. Two volumetric replacement ratios of
fly ash were studied. The modified Jander’s model was used
as a tool to quantify the kinetic coefficients of the pozzolanic
reaction at different temperatures.
superplasticizer was added while mixing with low speed for
90 seconds, followed by a further mixing stage with high
speed mixing for 120 seconds to ensure uniform dispersion of
the cementitious particles. After the mixing, the fly
ash-cement slurry was cast into cylindrical steel molds of 5
cm in diameter and 10 cm in height. The molds were
carefully sealed to prevent evaporation of water. After the
casting process, the paste specimens were cured at 20 oC, 35 oC, and 50 oC until the required ages were achieved.
When the required ages were reached, the hydration
reaction in the fly ash-cement pastes was stopped by crushing
the hardened specimens into pieces of about 3-5 mm, and
then immersing them in acetone for 24 h. After that the
samples were dried at 40 oC for 3 h, and placed in vacuum
desiccators for 2 days. The samples were then pulverized to
be used in the selective dissolution and the powder X-Ray
diffraction (XRD) measurements.
TABLE I: CHEMICAL COMPOSITION AND PHYSICAL PROPERTIES OF
PORTLAND CEMENT AND FLY ASH
Oxide (%) Portland Cement Fly Ash
SiO2 21.06 59.10
Al2O3 5.77 20.20
Fe2O3 2.67 8.85
CaO 63.55 3.36
MgO 1.85 1.17
SO3 2.28 0.12
TiO2 0.29 1.90
MnO 0.07 0.14
P2O5 0.25 2.58
Na2O 0.26 0.34
K2O 0.38 1.87
Blaine (cm2/g) 3,400 3,740
Density (g/cm3) 3.16 2.33
LOI (%) 0.614 0.960
TABLE II: MINERALOGICAL COMPONENTS OF PORTLAND CEMENT AND FLY
ASH USED IN THIS STUDY (OBTAINED BY THE XRD-RIETVELD METHOD)
Mineral (%) Portland Cement Fly Ash
C3S 64.54 -
C2S 14.04 -
C3A 4.40 -
C4AF 9.16 -
Mulite - 7.09
Quartz - 8.02
Hematite - 1.23
Glass content - 81.97
TABLE III: MIXTURE PROPORTIONS
Mix w/b
(mass fraction)
Fly ash/Cement
(vol. fraction)
Superplasticizer a)
(% of binder weight)
FA25 0.25 25/75 1.30
FA50 0.25 50/50 0.65
a) Content of superplasticizer that controls the flow diameter of paste at 200
mm
C. Method for Quantifying the Degree of Pozzolanic
Reaction of Fly Ash
The selective dissolution method was used in quantifying
the degree of pozzolanic reaction (d.o.p.) of fly ash. One
gram of hydrated fly ash-cement power was dissolved in 30
cm3 of an acid solution of 2N HCl at 60 oC for 15 min. The
undissolved portion was washed with hot water 3 times
before it was further dissolved in a base solution of 5%
NaCO3 at 80 oC for 20 min. The final residue was again
washed with hot water 3 times and then dried at 105 oC for 24
hours. The degree of fly ash reaction was calculated from
weight fraction of the reacted fly ash. Calculation details and
accuracies of this method can been seen in Termkhajornkit et
al. [9], which reported the above measurement conditions to
give a high accuracy of the amount of the reacted fly ash.
D. Method for Quantifying the Ca(OH)2 Content
The content of calcium hydroxide (Ca(OH)2) can be used
as an indicator for monitoring the progress of the pozzolanic
reaction of fly ash. The Ca(OH)2 content in hydrated fly
ash-cement sample was measured by the powder
XRD-Rietveld method. Samples used in the measurements
were the hydrated paste powder mixed with 10 wt. % of
corundum as a reference compound. The measurement
conditions of the XRD were a scanning range of 2 from 5o to
70o at 40kV, 20mA, step width 0.02o, and a scanning speed of
2o/min. The measured XRD data were further analyzed using
the Seroquant software which works based on the Rietveld
quantitative phase analysis.
III. RESULTS AND DISCUSSIONS
A. Effect of Curing Temperature
The degree of pozzolanic reaction (d. o. p.) of fly ash and
the Ca(OH)2 content at different curing temperatures are
plotted as a function of time in Fig. 1(a) for FA25 and Fig. 1
(b) for FA50. Small fluctuation in the progress of the d. o. p.
data may be observed, when the reaction rate is slow. This
demonstrates difficulties of the selective dissolution method
in detecting small change of a slow chemical reaction.
However, the overall kinetic tendency is the main focus in
this study. Each data point represents an average value of
three replicated samples. The maximum standard deviations
are found to be lower than 0.005 for the d. o. p. and below
0.032 for the content of Ca(OH)2. To avoid a chaotic
presentation, error bars are not included in the Figs.
Regarding the effect of curing temperatures in FA25
pastes, the pozzolanic reaction of fly ash is strongly
temperature-dependent (Fig. 1(a)). The onsets of the reaction
appear early at elevated curing temperatures. These points
are approximately 12 hr at 50 oC, 72 hr at 35 oC, and 672 hr at
20 oC curing, corresponding well with the peak of Ca(OH)2
evolution curves. After the onset, the pozzolanic reaction of
fly ash in pastes at different temperatures proceeds with
asimilar reaction rate. However, at 50 °C, the reaction rate
32
International Journal of Chemical Engineering and Applications, Vol. 5, No. 1, February 2014
At 20 oC, the d. o. p of fly ash reported by Lam et al. [3]
(solid rhombus marks) are also plotted for comparisons.
These data were measured from fly ash cement pastes with
w/b ratio of 24 wt.%, and at fly ash replacement ratios of 25
vol.% in Fig. 1(a) and 45 wt.% in Fig. 1(b)), which were
slightly different from the test condition used in this work. It
can be seen that the development trend of the pozzolanic
reaction of fly ash obtained by [3] are closed to the results of
this study, although the former is higher at some point. This is
basically due to differences in mix proportions, and qualities
of fly ashes and cements used. Nevertheless, it may be
deduced from this comparison that the d. o. p. of fly ash
obtained in this study is in a reliable tendency.
tends to slow down at later aging times. On the other hand,
the Ca(OH)2 contents decrease continuously after their peaks.
This indicates that the Ca(OH)2 is consumed by the
pozzolanic reaction of fly ash [10]. The final d. o. p. at the
end of observation (2160 hours or 90 days) are 0.22, 0.37,
and 0.43 for pastes cured at 20 °C, 35 °C, and 50 °C,
respectively. It is interesting to note that the d. o. p. in the
sample cured at 50 °C for 72 h (3 days) is almost the same as
that in the sample cured at 20 °C for 2160 h (90 days).
In both FA25 and FA50 pastes at 35 °C and 50 °C curing,
the rises of Ca(OH)2 evolutions at early stage occur earlier
than that at 20 °C. This can be attributed to the accelerated
hydration reaction of the cement due to an increased curing
temperature. The higher the curing temperature, the faster the
hydration of cement accelerates, and the faster the Ca(OH)2 is
produced.
Based on the presented experimental results, it can be
confirmed that the early kinetics of pozzolanic reaction of fly
ash is strongly temperature-dependent. In particular, the
onset of the reaction is activated earlier at high curing
temperatures. The higher the curing temperature and the
lower the fly ash replacement ratio, the higher is the d. o. p. at
the same ages. This suggests that at high curing temperatures
the pozzolanic reaction of fly ash plays a crucial role on the
microstructure development of fly ash concrete since early
age.
B. Kinetics Analysis
t.KN
f 3 11 (1)
KloglogNtlog f 3 11 (2)
where K is the reaction rate constant; N is the constant
describing the reaction mechanism; f is the degree of
pozzolanic reaction (d. o. p) obtained from experiment, and
t is the reaction time.
This equation is capable of classifying reaction processes
by considering on values of the exponent N . According to
[11]-[13], if N ≤1; the reaction is controlled by dissolution
processes, or by the immediate reaction at the surface of the
grains. If N >1; the reaction is controlled by the diffusion of
reactants through a layer of reaction products. This layer is
porous if N ≤2 and dense if N >2.
Eq.(1) can be written in an alternative form as shown in
Eq.(2). Based on the experimental tf data, Eq.(2) is
plotted as shown in Fig. 2(a) and Fig. 2(b). It can be seen that
the plot appears to be several linear portions with various
slopes, suggesting that the mechanism of pozzolanic reaction
varies with time and temperature. Thus, the coefficients K
and N in Eq.(2) are determined for each reaction stage and
they are listed in Table IV.
0
2
4
6
8
10
12
0.0
0.2
0.4
0.6
0.8
1.0
1 10 100 1000 10000
Ca(O
H) 2
(% w
t.)
Deg
ree o
f p
ozzo
lan
ic r
eacti
on
, d
.o.p
. (-
)
time (hr)
(a) FA25
D.o.p-20C D.o.p-35CD.o.p-50C D.o.p-20C Lam et al [3]Ca(OH)2-20C Ca(OH)2-35CCa(OH)2-50C
0
2
4
6
8
10
12
0.0
0.2
0.4
0.6
0.8
1.0
1 10 100 1000 10000
Ca(O
H) 2
(% w
t.)
Deg
ree o
f p
ozzo
lan
ic r
eacti
on
, d
.o.p
. (-
)
time (hr)
(b) FA50
D.o.p-20C D.o.p-35CD.o.p-50C D.o.p-20C Lam et al.[3]Ca(OH)2-20C Ca(OH)2-35CCa(OH)2-50C
Fig. 1. Kinetics of pozzolanic reaction of fly ash at different curing
temperatures; (a) FA25 paste, (b) FA 50 paste (d. o. p = degree of pozzolanic
reaction)
C. Effect of Curing Temperature on Reaction Mechanisms
The results in Table III show that the exponent N of Eq.(2)
changes with time and curing temperature. In general, the
reaction kinetics can be divided into 3 stages: stage I with
N >2.0, stage II with 1.0< N <2.0, and stage III with N >2.0.
The reaction kinetics in stage I is found as the initial period
of 20 oC and 35 oC curing with N =2.62 and 2.81 in FA25
paste, and with N =5.10 and 2.60 in FA50 paste, respectively.
33
International Journal of Chemical Engineering and Applications, Vol. 5, No. 1, February 2014
For FA50 pastes in Fig. 1(b), a similar temperature
dependency of the onset of the pozzolanic reaction, as found
in FA25 pastes, is observed. The peaks of the Ca(OH)2
curves are also closed to the onsets of reaction. However, the
rate and the extent of reaction at the same curing period are
lower. These are consistent with previous reports [3], [5], [6],
[10], which found that the d. o. p. of fly ash in pastes
containing high volume fly ash replacement is lower than that
in pastes with low fly ash content. Here, the final degree of
pozzolanic reaction in FA50 are 0.15, 0.23, and 0.27 for paste
cured at 20 °C, 35 °C, and 50 °C, respectively.
In this paper, a fly ash particle is modeled as a sphere
covered with product layer (C-S-H). The thickness of the
product layer increases with the d. o. p. Base on this
assumption, the pozzolanic reaction of fly ash is assumed to
be controlled by diffusion of reactants through the product
layer. The modified Jander's equation in Eq.(1) can be used to
analyze this behavior [11]-[13].
The higher the curing temperature, the shorter is the duration
of this stage. For 50 oC curing, it is expected that the reaction
has already progressed to the more advanced stage, therefore
the reaction kinetics in stage I is not observed. According to
criteria defined previously [11]-[13], the mechanism of
pozzolanic reaction in stage I should be controlled by
diffusion of ions through a dense product layer around fly ash
particle. However, Fig. 1 shows that at early period the main
pozzolanic reaction of fly ash in paste cured at 20 oC and 35 oC has not started. According to Fraay et al. [4], a long
dormant period of pozzolanic reaction of fly ash is because
the glass structure of the fly ash particle has probably not yet
dissolved. Therefore, the high N value in this case indicates
a slow diffusion of reactants (ions) through the dense glass
wall of the fresh fly ash particles rather than the dense C-S-H
product.
TABLE IV: KINETIC COEFFICIENTS OF THE MODIFIED JANDER’S MODEL
Mixture Temperature Exponent N Reaction rate constant K [day-1]
Stage I Stage II Stage III Stage I Stage II Stage III
FA25 20 oC 2.62 1.10 - 4.83 E-06 1.00E-03 -
35 oC 2.81 1.16 3.31 6.20E-05 4.18E-03 1.87E-05
50 oC - 1.58 7.38 - 6.10E-03 2.89E-08
FA50 20 oC 5.10 1.75 - 3.81E-05 1.37E-04 -
35 oC 2.60 1.56 3.64 5.22E-05 2.80E-03 1.21E-06
50 oC - 1.49 6.50 - 4.82E-03 3.68E-09
-2.5
-2
-1.5
-1
-0.5
0
-0.5 0 0.5 1 1.5 2 2.5
Lo
g(1
-(1-
f)1
/3)
Log(t) (day)
(a) FA25 50C
35C
20C
-2.5
-2
-1.5
-1
-0.5
0
-0.5 0 0.5 1 1.5 2 2.5
Lo
g(1
-(1-
f)1
/3)
Log(t) (day)
(b) FA50 50C
35C
20C
Fig. 2. Illustration of the plots of the kinetic function in Eq.(2) ; (a) FA25
paste, (b) FA50 paste
The reaction kinetics in stage II is observed for the initial
period of 50 oC and the middle period of 20 oC and 35 oC
curing in both pastes, as indicated by 1.0< N <2.0. According
to [11], the product layer around the fly ash particle is still
porous, and the pozzolanic reaction is, therefore, controlled
by diffusion of ions through porous product layer. The degree
of pozzolanic reaction increases largely with rapid reaction
rate, considering as the main stage of pozzolanic reaction.
The reaction kinetics in stage III is found at the last period
of 35 oC and 50 oC curing in both pastes, as shown by N >2.0.
The diffusion of ions through the dense C-S-H layer becomes
the rate controlling mechanism. This implies that at elevated
curing temperatures the product layer becomes denser
compared with 20 oC curing. As a result, the rate of
pozzolanic reaction is slowed down at later age. The dense
reaction product has also been found around the cement grain
when it hydrates at high curing temperatures [14], [15].
D. Effect of Curing Temperature on Reaction Rate
Table IV shows that the reaction rate constants ( K )
generally increase with curing temperatures, and decrease
with fly ash replacement ratios. Furthermore, the reaction
rate constant varies with reaction stages. The reaction in stage
II is the most active, showing the highest rate constant. In
stage I and stage III, the rates of reaction are relatively low.
This is a result of difference in reaction mechanisms, as
discussed in previous section.
The effect of curing temperature on the reaction rate
constant ( K ) in stage II is illustrated in Fig. 3. It can be seen
that the reaction rate increases linearly with increasing curing
temperatures. Similar tendency is observed in both FA25 and
FA50 pastes.
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
10 20 30 40 50 60
K (d
ay
-1)
Temperature [C]
Stage II
FA25
FA50
Fig. 3. Variations of the reaction rate constant with curing temperatures
II II
II III
III
I
I
II
III
I
II
I
II
III
34
International Journal of Chemical Engineering and Applications, Vol. 5, No. 1, February 2014
IV. CONCLUSIONS
From the results of this study, the following conclusions
can be drawn.
1) The kinetics of pozzolanic reaction of fly ash in low w/b
fly ash-cement pastes can be divided into 3 stages: First,
a slow diffusion through the dense glass wall of the fresh
fly ash particles, Second, a rapid diffusion through
porous product layer, and Third, a slow diffusion
through a dense product layer.
2) At a specific age, the degree of pozzolanic reaction of fly
ash depends on replacement ratio of fly ash and curing
temperature. The lower the fly ash replacement ratio and
the higher the curing temperature, the higher is the
degree of pozzolanic reaction of fly ash.
3) The rate and mechanism of pozzolanic reaction of fly ash
are strongly temperature-dependent. Elevating curing
temperature from 20 oC to 50 oC shortens the dormant
period and brings about faster the onset of the main
reaction from 28 days to 12 hr. The rate constant of the
main reaction (stage II) increases linearly with
temperature. However, the reaction rate in the later
period is retarded.
4) In future research, microstructural characteristics and
mechanical properties of hardened fly ash cement pastes
cured at different temperatures and different ages should
be measured and verified with the degree of pozzolanic
reaction of fly ash.
ACKNOWLEDGMENT
The authors gratefully thank to The Hitachi Scholarship
Foundation, Hokkaido University, and King Mongkut’s
University of Technology Thonburi, for their financial and
technical supports to this research.
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Mongkhon Narmluk received his B. S. Ind. Ed. and
M. Eng. in Civil Engineering from King Mongkut’s
University of Technology Thonburi (KMUTT),
Thailand, and Ph.D. in Solid Waste, Resources, and
Geoenvironmental Engineering from Hokkaido
University, Japan. He is currently a lecturer in the
Department of Civil Technology Education, Faculty
of Industrial Education and Technology, KMUTT.
His research interests include cement hydration,
utilization of solid wastes in concrete industry, and development of
eco-construction materials.
35
International Journal of Chemical Engineering and Applications, Vol. 5, No. 1, February 2014
[15] K. O. Kjellsen, R. J. Detwiler, and O. E. Gjorv, “Pore structure of
plain cement paste hydrated at different temperatures,” Cement and
Concrete Research, vol. 20, pp. 927-833, 1990.
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